The pilot indicated that prior to his fuelling the centre tank, the tank contained an unknown minimal amount of fuel. Using a minimum value of 50litres (13.25USgallons) of fuel added by the pilot prior to the flight, and a flight time of approximately 18 minutes to the time of the power loss, it was calculated that the engine would need to burn a minimum of 44USgph to empty the centre tank. Because the average fuel burn rate on segments of the first two flights of the day was calculated to be close to the stated aircraft flight manual rate of 25USgph, a fuel burn rate of 44USgph was unlikely. Therefore, illumination of the fuel pressure warning light and subsequent loss of engine power was not a result of the centre fuel tank running dry. The only two anomalies found in the fuel delivery system that could account for the loss of engine power was a restriction in the inline fuel check valve at the wobble pump location and a kink in the main engine fuel feed line. A restriction in the check valve at the wobble pump location would probably not result in an engine power loss, as fuel can still be drawn through the internal check valves in the wobble pump, providing an additional path of fuel to the engine. However, the path would have resulted in additional line losses due to the additional routing and check valves. The prior problem with the wobble handle moving up and down indicates that the inline fuel check valve was restricted to the extent that fuel was being drawn through the wobble pump to compensate for the restriction. The subsequent lowering of the fuel pump pressure likely lowered the flow of fuel through the check valve to a point where the fuel did not have to be bypassed through the wobble pump. The main fuel line was in an airframe compartment just aft of the engine firewall and behind the oil cooler temperature regulating valve. This area of the compartment was subjected to high temperatures during this flight, as the oil cooler would have been flowing at high capacity to dissipate heat from the hot engine oil. The result would have been heating of the fuel line and the fuel flowing through it. The kink in the fuel would have restricted the flow of fuel through the line causing a localised drop in fuel pressure at the narrowest point (venturi effect). Because of the location of the fuel tanks, there is very little head pressure in the tank to force the fuel forward. As well, suction in the fuel line was lower than normal because of the adjustment to the engine-driven fuel pump. As the tank level decreased and the fuel line temperature rose, it is likely a fuel vapour lock occurred. The boiling point of the fuel at the location of the kink would have been lowered because of the drop in pressure caused by the venturi effect. The kink, located at a bend in the line, would also have induced a turbulence in the fuel where the fuel flow over the bend would have separated from the internal wall of the line, downstream of the kink. As the fuel passing over this low pressure area was heated by the hot air being dissipated from the oil cooler, the fuel likely reached a temperature at which it vaporized. With the vaporization of the fuel, the engine-driven pump would have eventually cavitated, resulting in illumination of the fuel pressure warning light and loss of engine power. The hand-operated wobble pump would have been ineffective as the wobble pump was located downstream of the vapour lock and would have quickly lost its prime. Changing to the front tank would initially have had little effect on the operation of the engine, as the fuel could not have been sucked forward from the tank until the vapour lock cleared and either the engine-driven or hand-operated wobble pump regained its prime. A stiffer-than-normal return spring in the inline fuel check valve could also have delayed the repriming of the pumps. It is not known when or how the kink in the fuel line was made. Because of the severity of the kink, it is unlikely that the aircraft had been operating for very long in this condition, because a power loss would likely have happened earlier under similar conditions. The aircraft performance was degraded by the high outside air temperature and the drag of the externally loaded canoes. Because of the degraded performance and the pilot's concern about the engine operating temperature, the pilot was forced to level off at a lower than normal cruising altitude. This reduced the likelihood of a successful forced landing and the time available to restart the engine after it stopped. The information presented in the CBAAC0209 Advisory Circular and in the exemption to CAR703.25 document, if not contradictory is, at least, ambiguous. The purpose paragraph in the CBAAC 0209 Advisory Circular says that Type Certificate and STC authorization is not required to carry passengers with an external load. The purpose and application statement in the exemption to CAR703.25 document also says that Type Certificate and STC authorization is not required to carry external loads, but makes no reference to the carriage of passengers. The exemption document goes on to say that if an operator already has an STC or Type Certificate approval to carry external loads, the exemption does not apply. The ambiguous wording, and the degree of variance in how it is interpreted by Transport Canada inspectors, may contribute to an operator's inadequate understanding of its requirements and application. The exemption to CAR703.25 was not applicable to the deHavilland Beaver because carriage of external loads on the Beaver aircraft is governed by the information in the aircraft Type Certificate. If company personnel believed that the exemption did apply to their operation, they had not amended the operations manual to include the conditions listed in CBAAC0209, which also precluded them from taking advantage of it. Because it was common practice by the company to carry external loads with passengers, the pilot believed that it was acceptable, and expected of him, to do so. Because CBAAC 0209 contains no quantity and dimensional limitations, there is no regulatory defence in place to prevent an operator from attempting to carry large, non-aerodynamic external loads and subsequently exceeding the performance capabilities of their aircraft.Analysis The pilot indicated that prior to his fuelling the centre tank, the tank contained an unknown minimal amount of fuel. Using a minimum value of 50litres (13.25USgallons) of fuel added by the pilot prior to the flight, and a flight time of approximately 18 minutes to the time of the power loss, it was calculated that the engine would need to burn a minimum of 44USgph to empty the centre tank. Because the average fuel burn rate on segments of the first two flights of the day was calculated to be close to the stated aircraft flight manual rate of 25USgph, a fuel burn rate of 44USgph was unlikely. Therefore, illumination of the fuel pressure warning light and subsequent loss of engine power was not a result of the centre fuel tank running dry. The only two anomalies found in the fuel delivery system that could account for the loss of engine power was a restriction in the inline fuel check valve at the wobble pump location and a kink in the main engine fuel feed line. A restriction in the check valve at the wobble pump location would probably not result in an engine power loss, as fuel can still be drawn through the internal check valves in the wobble pump, providing an additional path of fuel to the engine. However, the path would have resulted in additional line losses due to the additional routing and check valves. The prior problem with the wobble handle moving up and down indicates that the inline fuel check valve was restricted to the extent that fuel was being drawn through the wobble pump to compensate for the restriction. The subsequent lowering of the fuel pump pressure likely lowered the flow of fuel through the check valve to a point where the fuel did not have to be bypassed through the wobble pump. The main fuel line was in an airframe compartment just aft of the engine firewall and behind the oil cooler temperature regulating valve. This area of the compartment was subjected to high temperatures during this flight, as the oil cooler would have been flowing at high capacity to dissipate heat from the hot engine oil. The result would have been heating of the fuel line and the fuel flowing through it. The kink in the fuel would have restricted the flow of fuel through the line causing a localised drop in fuel pressure at the narrowest point (venturi effect). Because of the location of the fuel tanks, there is very little head pressure in the tank to force the fuel forward. As well, suction in the fuel line was lower than normal because of the adjustment to the engine-driven fuel pump. As the tank level decreased and the fuel line temperature rose, it is likely a fuel vapour lock occurred. The boiling point of the fuel at the location of the kink would have been lowered because of the drop in pressure caused by the venturi effect. The kink, located at a bend in the line, would also have induced a turbulence in the fuel where the fuel flow over the bend would have separated from the internal wall of the line, downstream of the kink. As the fuel passing over this low pressure area was heated by the hot air being dissipated from the oil cooler, the fuel likely reached a temperature at which it vaporized. With the vaporization of the fuel, the engine-driven pump would have eventually cavitated, resulting in illumination of the fuel pressure warning light and loss of engine power. The hand-operated wobble pump would have been ineffective as the wobble pump was located downstream of the vapour lock and would have quickly lost its prime. Changing to the front tank would initially have had little effect on the operation of the engine, as the fuel could not have been sucked forward from the tank until the vapour lock cleared and either the engine-driven or hand-operated wobble pump regained its prime. A stiffer-than-normal return spring in the inline fuel check valve could also have delayed the repriming of the pumps. It is not known when or how the kink in the fuel line was made. Because of the severity of the kink, it is unlikely that the aircraft had been operating for very long in this condition, because a power loss would likely have happened earlier under similar conditions. The aircraft performance was degraded by the high outside air temperature and the drag of the externally loaded canoes. Because of the degraded performance and the pilot's concern about the engine operating temperature, the pilot was forced to level off at a lower than normal cruising altitude. This reduced the likelihood of a successful forced landing and the time available to restart the engine after it stopped. The information presented in the CBAAC0209 Advisory Circular and in the exemption to CAR703.25 document, if not contradictory is, at least, ambiguous. The purpose paragraph in the CBAAC 0209 Advisory Circular says that Type Certificate and STC authorization is not required to carry passengers with an external load. The purpose and application statement in the exemption to CAR703.25 document also says that Type Certificate and STC authorization is not required to carry external loads, but makes no reference to the carriage of passengers. The exemption document goes on to say that if an operator already has an STC or Type Certificate approval to carry external loads, the exemption does not apply. The ambiguous wording, and the degree of variance in how it is interpreted by Transport Canada inspectors, may contribute to an operator's inadequate understanding of its requirements and application. The exemption to CAR703.25 was not applicable to the deHavilland Beaver because carriage of external loads on the Beaver aircraft is governed by the information in the aircraft Type Certificate. If company personnel believed that the exemption did apply to their operation, they had not amended the operations manual to include the conditions listed in CBAAC0209, which also precluded them from taking advantage of it. Because it was common practice by the company to carry external loads with passengers, the pilot believed that it was acceptable, and expected of him, to do so. Because CBAAC 0209 contains no quantity and dimensional limitations, there is no regulatory defence in place to prevent an operator from attempting to carry large, non-aerodynamic external loads and subsequently exceeding the performance capabilities of their aircraft. The main engine fuel feed line had been damaged and kinked at some unknown time prior to the occurrence, which restricted the flow of fuel through the line. The damage to the fuel line was not discovered during regular maintenance inspection checks of the aircraft, which allowed a potentially serious problem to go unchecked. The area of the kink in the fuel line was in a hot area behind the engine oil cooler, which likely led to a fuel vapour lock situation resulting in a loss of engine power. The aircraft was equipped with a non-approved fuel check valve that did not meet the manufacturer's flow specifications. The fuel check valve not only presented a restriction in the fuel delivery to the engine, but was equipped with a stiffer valve plate return spring, which could delay the restarting of the engine if a fuel tank were to be run dry. The aircraft's load and the ambient weather conditions limited the cruising altitude for the flight. This reduced the likelihood of a successful forced landing and the time available to restart the engine after it stopped.Findings as to Causes and Contributing Factors The main engine fuel feed line had been damaged and kinked at some unknown time prior to the occurrence, which restricted the flow of fuel through the line. The damage to the fuel line was not discovered during regular maintenance inspection checks of the aircraft, which allowed a potentially serious problem to go unchecked. The area of the kink in the fuel line was in a hot area behind the engine oil cooler, which likely led to a fuel vapour lock situation resulting in a loss of engine power. The aircraft was equipped with a non-approved fuel check valve that did not meet the manufacturer's flow specifications. The fuel check valve not only presented a restriction in the fuel delivery to the engine, but was equipped with a stiffer valve plate return spring, which could delay the restarting of the engine if a fuel tank were to be run dry. The aircraft's load and the ambient weather conditions limited the cruising altitude for the flight. This reduced the likelihood of a successful forced landing and the time available to restart the engine after it stopped. Commercial and Business Aviation Advisory Circular (CBAAC)0209 does not place any finite restrictions for the carriage of external loads. There is a risk that operators may attempt to carry large, non-aerodynamic external loads and subsequently exceed the performance capabilities of their aircraft. Canadian Aviation Regulation (CAR)703.25 exempt aircraft are less restricted in their carriage of external loads than are non-exempt aircraft. This exposes exempt aircraft to a higher level of risk than non- exempt aircraft. Although the downed aircraft was found in a relatively short time, the absence of an ELT decreased the occupants' chances of a prompt rescue. The company was operating as if the exemption to CAR703.25 was in effect for the company, although it was not; thus, the pilot thought that his aircraft was approved to carry the load involved, which resulted in a decreased level of aircraft performance. The aircraft was not equipped with an optional fuel boost pump. A boost pump, if in use, could have prevented a vapour lock situation, or, once selected on, could have cleared a vapour lock.Findings as to Risk Commercial and Business Aviation Advisory Circular (CBAAC)0209 does not place any finite restrictions for the carriage of external loads. There is a risk that operators may attempt to carry large, non-aerodynamic external loads and subsequently exceed the performance capabilities of their aircraft. Canadian Aviation Regulation (CAR)703.25 exempt aircraft are less restricted in their carriage of external loads than are non-exempt aircraft. This exposes exempt aircraft to a higher level of risk than non- exempt aircraft. Although the downed aircraft was found in a relatively short time, the absence of an ELT decreased the occupants' chances of a prompt rescue. The company was operating as if the exemption to CAR703.25 was in effect for the company, although it was not; thus, the pilot thought that his aircraft was approved to carry the load involved, which resulted in a decreased level of aircraft performance. The aircraft was not equipped with an optional fuel boost pump. A boost pump, if in use, could have prevented a vapour lock situation, or, once selected on, could have cleared a vapour lock. The wording of the exemption to CAR703.25 is ambiguous. If not clarified and quantified, it is likely that industry and Transport Canada officials will continue to misinterpret its application and purpose. The pilot did not know or detect, prior to the flight, that the emergency locator transmitter (ELT) was missing.Other Findings The wording of the exemption to CAR703.25 is ambiguous. If not clarified and quantified, it is likely that industry and Transport Canada officials will continue to misinterpret its application and purpose. The pilot did not know or detect, prior to the flight, that the emergency locator transmitter (ELT) was missing. The examination and testing of the inline fuel check valves indicated that one valve did not meet the original component manufacturer's flow specifications, and as such was unapproved. Transport Canada has sent a letter to the airframe manufacturer requesting their position on this potential safety issue and what corrective action will be taken. The aircraft manufacturer's airworthiness department has taken the issue of the unapproved inline fuel check valve under advisement. Transport Canada has also established an Internal Working Group to review the issues surrounding the carriage of external loads.Safety Action The examination and testing of the inline fuel check valves indicated that one valve did not meet the original component manufacturer's flow specifications, and as such was unapproved. Transport Canada has sent a letter to the airframe manufacturer requesting their position on this potential safety issue and what corrective action will be taken. The aircraft manufacturer's airworthiness department has taken the issue of the unapproved inline fuel check valve under advisement. Transport Canada has also established an Internal Working Group to review the issues surrounding the carriage of external loads.